The present invention relates to an electron gun for a color cathode ray tube, wherein the last accelerating electrode forming a major lens is improved.
Generally, a cathode ray tube has a panel and a funnel which form a vacuum envelope. Red, green, and yellow phosphors are formed on the inner surface of the panel as stripes or dots, and a shadow mask frame assembly is installed inside the envelope. Also, a cylindrical neck is provided at the rear end of the funnel, so that an electron gun is stored within the neck, and a deflection yoke for deflecting electron beams emitted from the electron gun is mounted on the external surface of the funnel.
In the cathode ray tube constructed as above, electron beams of the red, green, and blue signals from the electron gun in the neck are passed through the shadow mask, thereby selectively landing on the phosphor layer. The quality of the picture formed by the landed electron beams is controlled by the size and shape of the focused electron beam spot and the converging accuracy of the three electron beams.
FIG. 1 is a schematic view of an electron gun described in U.S. Pat. Ser. No. 4,370,592 which is provided for improving the focus and convergence characteristics.
In the electron gun, cathodes 2, a control electrode 3, and a screen electrode 4 are provided constituting a triode for producing electron beams. A focus electrode 5 and an accelerating electrode 6 constitute a major lens system for the accelerating, focusing, and converging of the produced electron beams. The foregoing items are arranged sequentially with respect to the traveling direction of the electron beams. Focus electrode 5 has cup-shaped, first and second members 5a and 5b on the outgoing side of the electron beams facing accelerating electrode 6, and a third member 5c which aces screen electrode 4 located on the incoming side of the electron beams. Accelerating electrode 6 has cup-shaped, first and second members 6a and 6b on the incoming side of the electron beams.
First member 5a of focus electrode 5 and first member 6a of accelerating electrode 6 face each other in close proximity having horizontally elongated and common, large-caliber electron beam passing holes 5H and 6H, respectively. Second member 5b of focus electrode 5 has individual, small-caliber electron beam passing holes 5R, 5G, and 5B, and second member 6b of accelerating electrode 6 has individual, small-caliber electron beam passing holes 6R, 6G, and 6B.
Supplying respective voltages having a predetermined potential difference across focus electrode 5 and accelerating electrode 6 structured as above, forms an electrostatic lens for controlling the electron beams. However, since both large-caliber electron beam passing holes 5H and 6H have differing vertical and horizontal lines of symmetry, the electromagnetic fields focusing the outer electron beams become distorted, which results in unequal vertical and horizontal focusing effects on the electron beams as they pass through the common beam passing holes. Accordingly, the focusing characteristic of the electron beams is degraded due to the asymmetry of the electrostatic lens, so that the shape of an electron beam spot displayed on the screen is abnormally distorted.
Improvements for solving the above-described problems have been suggested in U.S. Pat. Nos. 4,370,592 and 4,388,552. Referring to FIG. 2, the shape of this electron gun is similar to the electron gun shown in FIG. 1, wherein an accelerating electrode 6 is formed by a first member 61 having a common large-caliber electron beam passing hole 6H and a second member 62 having individual electron beam passing holes 6R, 6G, and 6B.
Common electron beam passing hole 6H of first member 61 is somewhat peanut-shaped, wherein circular arc portions 6S and 6S' which are portions of virtual circles 6V and 6V', respectively, of a predetermined diameter or vertical width W2, are provided at both ends corresponding to outer electron beam passing holes 6R and 6B of second member 62, to protrusions 7 whose linear edges oppose to each other by an interval of a vertical width W1 which is smaller than the diameter (vertical width W2) of circular arc portions 6S, are arranged parallel to each other in the center of first member 61.
In accelerating electrode 6 having large-caliber electron beam passing hole 6H, apices 6a are formed at the points where circular portions 6S and 6S' at both ends of large-caliber electron beam passing hole 6H meet with protrusions 7. Thus, a length L along the fiat portions of protrusions 7 can be expressed by the following equation: EQU L=H-2R(1+cos .alpha.)
where the horizontal width of common electron beam passing hole 6H is designated by "H," the radius of each circular portion is "R," and the acute angle between a radius drawn from the center of either circular arc portion 6S or 6S' to an adjacent apex, and a horizontal line X-X', is ".alpha.."
The apex is sharp, and thus functions as a lightning rod by absorbing electric particles, so that the surrounding electric field distribution is abnormally distorted. Such distortion of the electric field distribution occurs within the region through which the electron beams pass. This is because the apex is adjacent to the electron beam passing region. Therefore, as illustrated in FIG. 2, the outer electron beams 81 and 83 (red and blue signals) passing through the outer electrostatic lens formed between focus electrode 5 and accelerating electrode 6 are attracted toward the sharp apices on which the electric field is concentrated, so that the sections of the electron beams become distorted into a triangular shape. When the electron beams having passed through the electrostatic lens are deflected toward the peripheries of the screen due to the deflection yoke, the electron beams are under the influence of severe astigmatism, and are thus distorted as shown in FIG. 3. At the left side of the screen, the spot of electron beam 83 (the blue signal) horizontally extends more severely than that of the red signal. Conversely, the spot of electron beam 81 of the red signal horizontally extends more severly than that of the blue signal at the right side of the screen. The difference in each signal electron beam spot degrades the color purity of the picture.